Concentrating photovoltaic electricity generation technology is widely accepted as an effective way to reduce cost of photovoltaic electricity generation. At present, a complete system for concentrating photovoltaic electricity generation mainly comprises a compound-eye concentrating photovoltaic assembly, a sun-tracker, and electric energy storage or inversion equipment. As a photo-electric conversion element, the compound-eye concentrating photovoltaic assembly comprises a compound-eye lens condenser and a circuit board installed with photovoltaic wafers.
The compound-eye lens condenser comprises a plurality of planar-arrayed condensing lens. During operation, the sun-tracker keeps the condensing lenses facing the sun perpendicularly for most of the time, then the condensing lenses focus the sun light and project it to the receiving surfaces of corresponding photovoltaic wafers on the circuit board to generate electric current in each of the photovoltaic wafer, then the electric current is exported by the circuits on the circuit board.
The concentrating solar cell assembly disclosed in patent application with disclosure number CN101640502A is very typical. The point-focusing Fresnel lens implemented in the assembly is widely recognized as the optimal option for condensing lens. There are additional references that disclose concentrating Fresnel lens as the condensing lens for concentrating photovoltaic electricity generation, such additional references are not included herein.
Actually, implementing Fresnel lens is not without shortcomings. For example, manufacturing defects on the surface texture of Fresnel lens cause loss in incoming light, resulting in a relatively low transmission rate of around 75%; such manufacturing defects are very hard to avoid under current technologies. As another example, Fresnel lens can be considered a combination of multiple co-axis convex lenses; as a result, the energy distribution of the focus light spot produced by Fresnel lens is not sufficiently uniform.
Replacing Fresnel lens with widely used ordinary spherical lens can solve the problem of low transmission rate. Spherical lens can only, however, focus light on the focal point of the lens, so no matter where the photovoltaic wafer is located, either in the front of the focal point, or further away from the focal point, uneven energy distribution will result on the receiving surface of the photovoltaic wafer, in the center and on the rim of the light spot, causing internal voltage difference in the wafer, producing internal current, such internal current is partially consumed inside the wafer, resulting in reduced output power from the wafer; additionally, internal current is the major cause for wafer internal temperature rise, while wafer internal temperature rises in term reduces efficiency of concentrating photovoltaic assembly.